We originally cloned SAFB1 as a transcriptional repressor, and it has since been implicated in a number of processes including hormone response, and transformation. The SAFB1 gene shows extremely high rates of loss of heterozygosity in breast cancer, and its loss is associated with high grade tumors, and reduced survival of breast cancer patients. Structure-function studies identified a transcriptional repression domain and overexpression of SAFB1 blunted estrogen-induction of gene expression and enhanced estrogen-mediated repression. Intriguingly, we found that sumoylation of SAFB1 was necessary for its co-repressor activity. We generated SAFB1-null mice which showed pre- and neonatal lethality, with surviving mice having dramatic defects in the reproductive system. To gain more insight into SAFB1's role in ER1 action, we performed an unbiased screen using SAFB1 siRNA knockdown followed by microarray analysis of estrogen-stimulated breast cancer cells. This experiment revealed a significant role for SAFB1 in estrogen-mediated repression of gene expression, and candidate genes were identified that showed a complete requirement for SAFB1 in estrogen-mediated downregulation. These candidates include proteins known to be involved in migration, invasion, and survival, such as NEDD9 and osteoprotegerin (OPG). Furthermore, we identified a novel role for HDAC7 in estrogen-mediated repression of some of the same candidate genes. We thus hypothesize that SAFB1 is an important and unique co-regulator that is critical for estrogen downregulation of genes, and that the repression is mediated through its posttranslational modification by sumoylation and, at least in part, through interaction with HDAC7. Finally, we hypothesize that SAFB1-mediated repression of its target genes is critical for its role in breast tumorigenesis, since loss of repression might result in increased migration, invasion, and survival of breast cancer cells. We will test these hypotheses by analyzing how SAFB1 mediates transcriptional repression of estrogen-responsive target genes, how is its co-repressor function modified by posttranslational modification, and we will test whether repression of its target genes contributes to SAFB1's role in breast tumorigenesis? This work is highly significant since it will provide insight into fundamental mechanisms of SAFB1 action which may contribute to a change in the paradigm of ER1 action. While ER1 has mainly been studied as a transcriptional activator, the equally frequent estrogen-mediated repression of gene expression, and in particular the role of co-repressors like SAFB1 in this repression, is an understudied area. Most importantly, we will also learn whether and how this repression activity of SAFB1 is critical for its role in breast tumorigenesis. PUBLIC HEALTH RELEVANCE: SAFB1 is a transcriptional co-repressor, and it has since been implicated in a number of cellular processes including hormone, response, immortalization, apoptosis, and transformation. We have discovered that SAFB1 plays a significant role in estrogen- mediated repression of gene expression, which is an understudied yet critical process in estrogen receptor action. Here we propose to study the in-depth mechanism of this repression, and how this relates to SAFB1's role in breast tumorigenesis.